An electrically conductive coating film has a variety of uses of electromagnetic radiation shielding for cathode-ray tubes, plasma display panels etc., infrared ray shielding for construction materials or automobiles, coverage for preventing electrostatic charging of electronics and cell-phones, heating wires for defogging of glass, wirings on circuit boards etc., coatings for giving electric conductivity to resin, or a circuit itself. As methods of forming such electrically conductive coating films, there have been known conventionally methods such as a vacuum metal deposition method, a chemical vapor deposition method, an ion sputtering method and a method of applying a metallic colloid solution having metallic particles dispersed in a dispersion medium on a base material and then heat-calcining it. These methods have some problems such as cumbersome operation, low mass productivity, and necessity for heating at high temperatures. On the other hand, there have been conducted so far to form wirings on printed boards by etching an electrically conductive film or by using an electrically conductive paste. However, the etching method has problems such as troublesome operation, high cost and liquid waste disposal and is also undesirable from an environmental point of view. Further, the method of using the electrically conductive paste requires necessity for formation of electrically conductive circuits such as wirings by screen process printing which is problematic in productivity, and has another problem of heating at high temperatures after printing in order to attain excellent electric conductivity.
IC cards capable of noncontact reading and rewriting of information, such as banking cash cards, train passes, and various prepaid cards, appear recently in place of cards utilizing a contact-type magnetic recording system, and their demand is increasing. Further, IC tags using this noncontact communication system are also proposed and come into practical use. As the method of recognizing media utilizing this system capable of noncontact reading and rewriting information, that is, noncontact-type media, there are two system at present: an electromagnetic induction system and a radio wave system. In both the systems, media are composed of a IC chip and an antenna, and it is essential that the antenna portion that is an electrically conductive circuit is of low resistance for lowering loss at the time of transmitting and receiving radio waves. The antenna portion that is an electrically conductive circuit of noncontact-type media such as IC cards and IC tags is formed by a method of transferring a metal foil such as copper foil or aluminum foil to a base material or by a method of printing an etching-resist ink in an antenna circuit pattern on a metal foil laminated on a base material such as a plastic film and then etching it. A copper-line coil or a wire may be used as an antenna. However, any of these methods have limits of productivity and are unsuitable for mass production. Further, as described above, there is also a method of utilizing an etching method. The method, however, is not preferable from an environmental point of view because of having the problems such as a liquid waste disposal as described above, and is also problematic in respect of cost.
As another method of forming an electrically conductive circuit for noncontact-type media, a method of screen process printing an electrically conductive paste has been carried out. A polyester resin or an epoxy resin is mainly used as a binder in conventional electrically conductive pastes above, from which a conductive circuit having a volume resistivity on the order of 10−5 Ω·cm has been obtained (see Patent Documents 1 and 2). However, these conventional electrically conductive pastes require drying conditions of high temperatures of 100° C. or more and several minutes to several dozen minutes, as well as a coating thickness of about 10 μm or more after drying. The method, therefore, have limitations in consideration of cost and mass productivity for use in IC tags. Electrically conductive circuits formed from the conventional electroconductive pastes using a polyester resin or an epoxy resin as a binder have also a problem of low reliability under a high-temperature and high-humidity environment at 85° C. in 85% RH.
On the other hand, it is known recently that a volume resistivity on the order of 10−6 Ω·cm can be obtained with a relatively thin coating of about 0.1 to 5 μm in thickness by using silver nano-particles (see Non-Patent Document 1). For exhibiting this resistivity, however, sintering of the paste at 200° C. is necessary, thus making this method hardly applicable to usual paper base materials and plastic base materials such as polyester used in IC cards or IC tags. In addition, this method has a problem of inferior adhesiveness to a base material under a high-temperature and high-humidity environment.
There is also an attempt at reducing resistance without influencing a base material, which comprises combining an active energy ray-curable electrically conductive paste using urethane acrylate or the like as a binder with heat pressure roll treatment (see Patent Document 3). However, the resulting electrically conductive circuit has a volume resistivity on the order of 10−4 Ω·cm, which is too high to cope with a broad range of frequency used in an antenna circuit. There is also a problem of lack of reliability because of instable resistance under a high-temperature and high-humidity environment.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-260224    Patent Document 2: JP-A-2003-16836    Patent Document 3: JP-A-2001-64547    Non-Patent Document 1: Journal of Japan Institute of Electronics Packaging, Vol. 5, No. 6 (2002), pp. 523-528
Accordingly, an object of the present invention is to provide an electrically conductive ink utilizable in various uses and free of the conventional problems, an electrically conductive circuit using the electrically conductive ink and a noncontact-type medium having an electrically conductive circuit formed by using the electrically conductive ink, as well as a method of forming the electrically conductive circuit.
Specifically, an object of the present invention is to provide an electrically conductive ink, from which an electrically conductive coating that is highly reliable under a high-temperature and high-humidity environment can be formed at low temperatures in a short time.
Another object of the present invention is to provide an electrically conductive ink having the above feature and capable of forming a thin electrically conductive circuit having such a low volume resistivity on the order of 10−5 Ω·cm as to be usable as an antenna circuit for noncontact-type media.
Still another object of the present invention is to provide an electrically conductive circuit having a low volume resistivity on the order of 10−5 Ω·cm and being highly reliable under a high-temperature and high-humidity environment, and a method of forming the same.
A further object of the present invention is to provide a noncontact-type medium provided with an electroconductive circuit having a low volume resistivity on the order of 10−5 Ω·cm and being highly reliable under a high-temperature and high-humidity environment.